This invention relates generally to early detection of toxic and corrosive by-products formed by electrical discharges in some high voltage systems and more specifically to an ion-molecule reaction cell and negative ion mass spectrometry method for detecting by-products that result from degradation of sulfur hexafluoride, a gas insulator, and is a result of a contract with the United States Department of Energy.
Sulfur hexafluoride, SF.sub.6, is a gas used as an insulator in conjunction with solid insulating material in high voltage systems such as transmission lines, substations and switchgear. In the course of operating such high voltage systems electrical discharges can occur in the form of coronas, arcs or sparks. If there are no impurities present in the system, only SF.sub.6, then SF.sub.6 attaches to electrons to form SF.sub.6.sup.- or SF.sub.6 could degrade to SF.sub.4 and fluorine (F or F.sub.2). In either case an equilibrium is established between SF.sub.6 and SF.sub.6.sup.- or SF.sub.4 and F or F.sub.2. However, if there are impurities in the system the SF.sub.6 can further degrade eventually resulting in voltage breakdown. In the presence of oxygen or water from atmospheric moisture, silicon from lubricants and insulators, and tungsten from electrode materials, gaseous by-products are produced that can be toxic and corrosive. These by-products include SiF.sub.4, SOF.sub.4, SO.sub.2 F.sub.2, SO.sub.2, SOF.sub.2, WF.sub.6, SF.sub.4, F, F.sub.2 and HF.
Conventional methods for detecting these by-products include electron impact mass spectrometry (MS), gas chromatography (GC) with thermal conductivity detection, gas chromatography with electron capture detection, and a combination of gas chromatography and mass spectrometry. Conventional electron impact mass spectrometry has limited use since the mass spectra are dominated by the SF.sub.6 ion peaks, obscuring the ion peaks representative of the SF.sub.6 by-products. GC with thermal conductivity detection has sensitivities of only about 100 ppm. GC with electron capture detection is not suitable for some of the by-products since they are not good electron attachers and must compete with the strongly electron attaching SF.sub.6 ; for example, SiF.sub.4 does not attach low energy electrons and only weakly attaches electrons at high electron energies (.about.10 ev). Combination GC/MS has the problem of complex spectra and low sensitivity (.about.1 ppm). An analytical method that detects the presence of degradation produced by-products in the parts per billion (ppb) level would be preferable.
This invention detects these by-products at very low concentrations (ppb) using an ion-molecule reaction cell and a negative ion mass spectrometer. The ion-molecule reaction cell has previously been used in electron attachment studies that were limited to studying parent ions but were not used to generate ions other than the parent ions, as is done in this invention. Negative ion mass spectrometry has not been previously used to detect by-products of SF.sub.6 degradation because generation of negative ions of the by-products has not been previously done.
There is a need to detect the formation of impurities as early as possible, preferably in the ppb range since prolonged buildup can result in degradation of SF.sub.6 and attendant high voltage breakdown and also because the by-products formed in the degradation process are, for the most part, toxic and corrosive, posing a hazard to system operators.